The present invention relates to a method for manufacturing semiconductor devices, and more particularly, to a method for depositing titanium layers on a substrate.
Device density in integrated circuits (ICs) is constantly being increased. To enable the increase in density, device dimensions are being reduced. As the dimensions of device contacts get smaller, device contact resistance increases, and device performance is adversely affected. Methods for decreasing device contact resistance in ICs are needed to obtain enhanced device and IC performance.
Device contacts with reduced resistance may be created by forming certain metals on a silicon semiconductor base layer. These metals react with the underlying silicon, for example, to form silicides. Silicide device contacts are desirable because they reduce the native oxide on silicon. The native oxide is undesirable because it increases the contact resistance.
In one embodiment, titanium is used to form silicide device contacts for two reasons. First, titanium silicide has superior gettering qualities. Also, titanium silicide forms low resistance contacts on both polysilicon and single-crystal silicon.
Titanium silicide device contacts are normally formed with the following process. First, a thin layer of titanium is formed on top of the silicon base layer, such as a substrate. The titanium adjoins active regions exposed by contact holes in an isolating layer, such as an oxide, above the silicon base layer. Then, the silicon base layer is annealed. As a result, the titanium reacts with the active regions of silicon to form titanium silicide.
However, because titanium cannot be readily deposited in a pure form, additional processing steps are required to form titanium silicide device contacts. Titanium precursors, such as titanium tetrachloride, are commonly available and can be used to form titanium. Titanium tetrachloride, though, can only be reduced at temperatures exceeding 1000 degrees Celsius with certain reducing agents. At these temperatures, the silicon base layer will be damaged. Therefore, there is a need for a method of forming titanium from titanium precursors at lower temperatures.
Furthermore, the resistance of device contacts can be adversely increased by conductive layers coupled between the device contacts and other components. The conductive layers may be formed by the same metal layer used to form the device contacts. As device dimensions shrink, the contact holes become relatively deeper and narrower. Also, the walls of the contact holes become steeper, and closer to vertical. As a result, most metal deposition techniques form conductive layers having relatively small step coverage, and hence relatively high resistance. Step coverage is the ratio of the minimum thickness of a film as it crosses a step, to the nominal thickness of the film on flat regions, where thickness is generally measured perpendicular to the surfaces of the step and flat regions, and where the resultant value is usually expressed as a percentage. Thus, the effective contact resistance is increased at lower values of step coverage. Therefore, there is also a need for a method of forming conductive layers having increased step coverage to reduce effective device contact resistance.
Conformal layers of titanium having good step coverage have been previously formed at lower temperatures with chemical vapor deposition. Such techniques are disclosed in U.S. Pat. Nos. 5,173,327, 5,273,783 and 5,278,100, which are hereby incorporated by reference. However, alternative, effective and efficient techniques for forming titanium films are desired.
The present invention provides a method, and a corresponding resulting structure, for forming conformal titanium films supported on a substrate of an integrated circuit (IC) by forming a seed layer supported by the substrate, and then reducing a titanium precursor with the seed layer. In one embodiment, the seed layer comprises a main group element selected from the group consisting of zinc, cadmium, mercury, aluminum, gallium, indium, tin, silicon, germanium, lead, arsenic and antimony. The seed layer is formed by combining a first precursor and a reducing agent by chemical vapor deposition (CVD). Then, titanium is formed by combining a second precursor with the seed layer by CVD.
In another embodiment, the present invention may further comprise the step of annealing the titanium to form titanium silicide.
In another embodiment, forming the seed layer further comprises forming a seed layer according to the following chemical process (I):
MRx+H2xe2x86x92M+alkanes, 
wherein:
M is a main group element selected from the group consisting of zinc, cadmium, mercury, aluminum, gallium, indium, tin, silicon, germanium, lead, arsenic and antimony;
R is an alkyl group; and
x is some integer value determined by the valence of M.
In one embodiment, chemical process (I) is performed at a temperature between approximately 100 and 600 degrees Celsius.
In yet another embodiment, the step of forming titanium further comprises the step of combining the seed layer with the second precursor that is titanium tetrachloride according to the following chemical process (II):
TiCl4+Mxe2x86x92Ti+MClx. 
In one embodiment, chemical process (II) is performed at a temperature between approximately 100 and 600 degrees Celsius.
In yet another embodiment, titanium may be formed in a single step according to the following chemical process (III):
TiCl4+M(source)xe2x86x92Ti+MClx 
In one embodiment, chemical process (III) is performed at a temperature between approximately 100 and 700 degrees Celsius.
In yet a further embodiment, the present invention may be an IC comprising a layer of a titanium alloy, coupled to a titanium silicide contact. In yet another embodiment, the present invention may be a memory comprising a memory array operatively coupled to a control circuit and an I/O circuit. The memory array, control circuit and I/O circuit comprise a layer of a titanium alloy coupled to titanium silicide contacts. In yet another embodiment, the titanium alloy may comprise titanium and an element selected from the group consisting of zinc, cadmium, mercury, aluminum, gallium, indium, tin, silicon, germanium, lead, arsenic and antimony. In still another embodiment, the titanium alloy may comprise titanium and zinc.
It is a benefit of the present invention that high step coverage metal layers can be formed. Further features and advantages of the present invention, as well as the structure and operations of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.